Astronomy

CONTENTS

4 ASTRONOMY • DECEMBER 2015

20
How immense is the
universe?
Unfettered by tales of science
fiction, the cosmos is almost
incomprehensibly large.
DAVID J. EICHER

24
Our solar system:
Realms of fire and ice
We start your tour of the cosmos
with gas and ice giants, a lot of
rocks, and the only known abode
for life. FRANCIS REDDY

30
The Milky Way:
Earth’s home galaxy
Giant clouds of gas and dust
sprinkled with splashy star clus-
ters adorn the Milky Way’s spiral
arms, while the galaxy’s vast halo
teems with darker matter.
FRANCIS REDDY

36

The Sky this Month

A gemlike shower.

MARTIN RATCLIFFE AND

ALISTER LING

38

StarDome and

Path of the Planets

RICHARD TALCOTT;

ILLUSTRATIONS BY ROEN KELLY

44

The Local Group:

Our galactic

neighborhood

The Milky Way’s family of galax-

ies is locked in a multi-billion-

year battle for gravitational

supremacy. KATHERINE KORNEI

50

The Virgo Supercluster:

Our 100,000 closest

galaxies

Small groups combine with larger

clusters to form a vast network

of interconnected galaxies that

spans 110 million light-years.

SARAH SCOLES

56

Limits of the cosmos:

The far reaches of space

Everything astronomers have

learned points to a universe

populated by superclusters of gal-

axies, incredibly long filaments,

and colossal voids. LIZ KRUESI

62

Ask Astro

Gathering the dark.

COLUMNS

Strange Universe 10

BOB BERMAN

For Your Consideration 14

JEFF HESTER

Observing Basics 64

GLENN CHAPLE

Secret Sky 66

STEPHEN JAMES O’MEARA

Cosmic Imaging 68

ADAM BLOCK

QUANTUM GRAVITY

Snapshot 9

Astro News 12

IN EVERY ISSUE

From the Editor 6

Letters 10, 14, 64

Web Talk 65

New Products 67

Advertiser Index 70

Reader Gallery 72

Breakthrough 74

FEATURES

Astronomy (ISSN 0091-6358, USPS 531-350) is pub-
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DECEMBER 2015

VOL. 43, NO. 12

Blogs from

the Local

Group

Insight from

the editors

The Sky

this Week

A daily digest

of celestial

events

Reader Photo

Gallery

Share your

astroimaging

efforts

50 Weirdest

Objects

Explore the

most curious

cosmic oddities

ONLINE

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Go to http://www.Astronomy.com

for info on the biggest news and

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informative videos, and more.

Visit Astronomy.com/toc

for bonus material — it’s

exclusive to Astronomy

magazine subscribers.

20

TONY HALLAS

ON THE COVER

The universe, at least 93 billion

light-years across, is awash in

some 100 billion galaxies like NGC

6946, a beautiful face-on spiral.

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